812. Respiration in plants refers to their refations to the atmosphere. So in animals. These relations are in either case vitally important.

813. Experiment. Place a small, healthy potted plant (sc. Geranium, Mimosa) under the receiver of an air-pump, and thoroughly exhaust the air. At once every vital process ceases - no absorption, no assimilation, no irritability, but speedily decay ensues. A vacuum would be no more fatal to a sparrow. Air is quite as necessary to the one as to the other.

814. Illustration. So also when only the roots are excluded from the air by being buried deeply in an embankment, the tree suffers injury and perhaps perishes.

815. Respiration in plants, or aeration (as sometimes called) consists of all those operations by which the sap is brought into contact with the air or subjected to its influence. It occurs in the intercellular passages, in the spiral vessels everywhere, but especially in the leaves and all other organs which have chlorophylle and stomata.

816. The vital importance of respiration is seen in the vast extent of the respiratory apparatus, consisting of millions of leaves and billions of breathing pores (stomata) and tracheae (vessels)!

817. The facts connected with respiration, which seem to have been well established by the experiments of Saussure, Garreau, Moue, Draper, etc., are these:

1. Carbonic acid (C O2) is absorbed by the leaves and all green tissues, under the direct solar light.

2. Oxygen (O) is absorbed by the leaves and all green tissues in the absence of direct solar light, and by the roots, flowers, fruits, and germinating seeds at all times.

3. The oxygen thus absorbed unites with some of the free (or nascent) carbon already in the tissues, and forms carbonic acid.

4. By a process of assimilation (§ 747) carbonic acid within the green tissues, from whatever source derived, is decomposed under the direct sunshine, and its carbon is retained; but

5. Its oxygen is set free and exhaled.

6. Carbonic acid is exhaled by the leaves and all the green tissues in the absence of the sunshine, and by all other parts (root, flowers, fruit, and germinating seeds) at all times. Hence it appears that there are

818. Two phases of aerial action constantly performed and seem-ingly opposed to each other. One dependent wholly upon the clear sunshine, in which, by the leaves, etc., C O2 is absorbed, decomposed, and O returned to the atmosphere; the other, in which O is absorbed, and C O2 exhaled, by the leaves in the absence of sunshine, and by all other parts (roots, flowers, etc.) at all times. Both are equally and vitally important.

819. The former process becomes visible to the eye by the rapid development of chlorophylle accompanying it, the latter by its gradual loss. Hence, during a protracted season of cloudy weather vegetation grows sensibly paler, but a few hours of sunshine restores the green to its wonted depth and richness.

820. Blanched plants. Hence, also, plants growing in constant darkness and shade, as potatoes in the cellar, are very pale, and manifest their affinity for light by stretching themselves with famishing eagerness towards the slender sunbeam which gains admittance. Analysis shows structures thus grown to be deficient in carbon. We may easily repeat the

821. Experiment of saussure. Place a quantity of freshly gathered leafy sterna under a bell-glass full of rain-water, and thus expose them to the sun. Soon bubbles of gas arise and slowly collect above, pure oxygen gas, as long ago proved by Dr. Priestly.

822. Repeat the experiment with boiled or distilled water, and no oxygen will appear. Rain-water contains C O2 in solution, boiled water does not. The 0 must therefore have come from the C O2 as would appear.

823. Experiment. Inclose air-tight in a glass globe the end of a leafy branch, without severing it from the tree. Thus it has been found by careful analysis after a day of sunshine that the proportion of 0 was increased at the expense of C O2 within the globe; and vice versa by night or in the shade.

824. The results of both transpiration and respiration, as concerns the plants, tend to concentrate the diluted sap by the elimination of the water, which served merely for its conveyance, and to assimilate it into food capable of being organized into cells and their various contents.

And it is proper in this place also to notice the effects of this vast machinery upon the constitution of the atmosphere and its relation to the animal kingdom.

825. Carbonic acid gas is dissolved in the atmosphere and somewhat uniformly diffused throughout its whole extent in the proportion of about 4 parts in 10,000, or 1/2250. This gas flows, and is ever flowing into the air from decaying animal and vegetable substances, from combustion, and from the breath of all living animals. The quantity thus added to the atmosphere annually is estimated at 100 billions lbs., or nearly one tenth of the whole amount of carbon, and yet it does not accumulate.

826. The demand and supply. Were we able to compute in pounds the annual growth of the entire plant world, and the proportion of solid carbon which enters into that amount, we should doubtless find that the grand total of the demand equals this grand total supply.

A poisonous atmosphere. And further; not only are the necessities of the plant met by this wonderful circulation, but the necessities of animal existence also. Carbonic acid is poisonous, and should it be left to accumulate unchecked, it would gradually corrupt the air, and within a few centuries extinguish all animal life.

828. Animals and plants mutually dependent. Thus are the two kingdoms of the organic world mutually, through the inorganic, dependent upon each other. The plant furnishes the oxygen which the animal consumes, the animal the carbonic acid which the plant consumes, while each would perish in an atmosphere of its own production. "Great and marvelous are thy works, O Lord of Hosts! in Wisdom hast thou made them all."